1. Field of the Invention
The present invention relates to a transmission unit and a failure recovery method. More particularly, the present invention relates to a transmission unit which has the capability of recovering from failure, and to a failure recovery method for that purpose.
2. Description of the Related Art
Recent years have seen an increasing amount of packet traffic because of the expanding use of the Internet. In today's Internet Protocol (IP) networks, the packets carry various types of information, from ordinary computer data to delay-sensitive realtime voice and video streams. To address the requirements for packet transport with smaller delays, a new traffic engineering protocol called “label switching” has been proposed. Label is a short fixed-length value that is attached to packets at the ingress node to specify a path to a particular destination. Without using ordinary layer-3 (network layer) routers, label-switched networks transport labeled packets over a layer-2 path that is previously defined for each destination. That is, the label switching mechanism enables fast packet delivery by eliminating upper-layer routing procedures. This new technology, called the multiprotocol label switching (MPLS) protocol, is currently under standardization by the Internet Engineering Task Force (IETF).
To ensure a high level of network availability, MPLS networks employ a failure recovery mechanism called “local repair,” for example. FIGS. 22 to 24 show a failure recovery process with local repair techniques. The illustrated network is constructed with a plurality of local switch routers (LSRs). Six LSRs 202 to 205, 207, and 208 are connected circularly, and two LSRs 201 and 206 are linked to the LSRs 202 and 205, respectively.
Suppose here that an end-to-end working path W is established between two LSRs 201 and 206 as shown in FIG. 22. Packets originating from the ingress LSR 201 are delivered to the egress LSR 206, being label-switched at each intermediate LSR on the working path W as follows:                LSR 201→LSR 202→LSR 203→LSR 204→LSR 205→LSR 206Protection paths p1 to p3 are provided to make this connection tolerant of possible failure with the links L1 to L3 along the path W. Those paths p1 to p3 would detour packets around the links L1 to L3, respectively. In the event that one of those links is disrupted, two LSRs located at both ends of that link would reconfigure themselves to activate an appropriate protection path, thereby recovering from the link failure.        
Referring to FIG. 23, suppose, for example, that the working path W has failed somewhere on the link L2. Upon detection of the problem, the LSRs 203 and 204 switch the failed link L2 to the protection path p2. Accordingly, a new packet route is established as follows:                LSR 201→LSR 202→LSR 203 (loopback)→LSR 202→LSR 207→LSR 208→LSR 205→LSR 204 (loopback)→LSR 205→LSR 206Packets are label-switched at each LSR on this new working path Wa, and finally reach the destination LSR 206 as shown in FIG. 24.        
The above-described conventional failure recovery technique, however, requires a lot of network resources to be allocated previously for protection purposes. In the example of FIGS. 22 to 24 discussed above, three protection paths p1 to p3 have to be reserved for one working path W, to provide an alternative route that detours around the link L1 to L3 in case of failure. This is very inefficient in terms of resource usage. While it surely contributes to the improved availability of networks, the conventional protection method causes a problem in operability and usability of communication services.